1. Field of the Invention
This invention relates generally to a battery charger monitor that monitors battery pack parameters during charging of a battery and, more particularly, to a battery charger monitor that monitors battery voltage, battery temperature and determines battery pack identification using an identification capacitor within the battery pack, where the battery pack identification and the battery temperature are monitored on a single line between the battery monitor and the battery pack.
2. Discussion of the Related Art
Many different electrical devices, such as some power tools, cellular phones, appliances, etc., use battery packs that are rechargeable. Modern battery chargers are capable of charging battery packs having different rated voltages and different battery chemistry, i.e., nickel-cadmium, metal hydride, lithium, etc. Most of these battery chargers monitor the voltage of the battery pack during the charging process to provide a full charge without exceeding the maximum charge of the battery pack which could cause damage to the pack. U.S. Pat. Nos. 4,388,582 and 4,392,101, both issued to Sar et al., and assigned to the assignee of the present invention, disclose a quick battery charging technique which monitors the charging of a battery pack by noting inflection points in a voltage charging curve as the electrical chemical potential within the battery cells changes with respect to time. By determining specific inflection points in the charging curve, it is possible to accurately terminate the charging when the battery pack is at full charge.
In addition to monitoring the voltage of a battery pack that is being charged, the battery monitor may also determine other pieces of useful information, including battery pack temperature and battery pack identification. Monitoring battery pack temperature is useful because overheating the battery pack during charging can cause significant damage to the battery pack. Determining the battery pack identification is desirable to ascertain certain things such as maximum battery voltage and capacity, battery chemistry, manufacturing date, etc. This information can be used by a charging control algorithm controlling the charging process to aid in monitoring and adjusting the charging process.
Battery chargers and charger monitors that monitor battery voltage and battery temperature during the charging process can be found in U.S. Pat. No. 5,352,969 and U.S. patent application Ser. No. 08/834,375, filed Apr. 16, 1997, titled "Indirect Thermal Sensing System for a Battery Charger," both assigned to the assignee of the instant application. Both of these documents disclose an analog-to-digital conversion technique that converts an analog battery voltage signal to a digital representation suitable for microprocessor analysis. The battery voltage signal is attenuated by a voltage attenuation or prescaler circuit, acting as a voltage divider, to cause the battery voltage to be in a predetermined narrow range regardless of the battery voltage. The prescaler circuit allows the battery charger to effectively monitor the battery voltage for a wide range of battery packs having varying voltages. U.S. patent application Ser. No. 08/834,375 also discloses a technique for monitoring battery temperature using a similar analog-to-digital conversion method.
U.S. Pat. No. 5,489,834 issued to Pitkanen discloses a battery type and temperature identification circuit. The battery pack being charged includes a negative temperature coefficient resistor, a voltage divider resistor, and a fixed identification resistor electrically connected in a voltage divider circuit. The identification resistor is different for each different battery pack type, and provides the basis for determining battery type. An output voltage from the voltage divider circuit follows a particular curve set by the identification resistor as the temperature on the pack changes. Because the value of the identification resistor is selected based on the battery pack type, and the values of the negative temperature coefficient resistor and voltage divider resistor are selected to be the same or nearly the same for each battery type, the different voltage curves at the output of the voltage divider circuit for the different battery types can be spaced apart. By determining which curve the output voltage follows for changes in temperature gives an indication of the battery pack identification.
This technique does, however, suffer from a number of drawbacks and disadvantages. Particularly, because each of the voltage signals for the different battery packs are voltage divided by a particular identification resistor to be separated, the resolution of the voltage signal is low, and distinction between one voltage curve to a next voltage curve could provide error because of curve overlap. Because one voltage curve for one battery pack type is adjacent to another battery pack type, it may be impossible to tell whether the voltage representation is at the end of one voltage curve or at the beginning of a next voltage curve. Additionally, because the battery type identification circuit must have accurate resolution within the range of each voltage curve as well as over the entire range of the voltage curves combined, a very expensive analog-to-digital converter is required.
German Patent Application No. DE 42 25 686 A1, published Mar. 3, 1994 discloses a circuit for recognizing and charging a battery pack, where the battery pack includes a temperature dependent resistor and an identification capacitor. A microprocessor applies a DC voltage signal to the battery pack to determine the resistive value of the temperature dependent resistor. Next, a square wave signal is applied to the battery pack, and the impedance is measured. The value of the identification capacitor is then determined by the equation Z=1/(sqr(1/R).sup.2 +(1/Xc).sup.2) to identify the battery pack.
What is needed is a battery charger monitor circuit that is able to charge a battery pack in the charger, monitor the voltage of the battery, monitor the temperature of the battery, and provide an indication of the battery pack identification in and efficient and cost effective manner, and doesn't suffer the drawbacks of the prior art devices discussed above. It is therefore, an object of the present invention to provide such a battery monitor circuit.